The overwhelming majority of patients usually recover, even without medical treatment, within a week of falling ill.

The WHO has said the pandemic is developing at such a high speed that it is now pointless to try to document every case.

In past pandemics, flu viruses have needed more than six months to spread as widely as the new H1N1 virus has spread in less than six weeks.

But officials have stressed that there is an ongoing need for all countries to monitor unusual events, such as clusters of severe or fatal cases, or unusual clinical patterns, closely.

Some 125,000 laboratory-confirmed cases have been reported worldwide - but the number of actual cases far exceeds that.

WHO spokeswoman Aphaluck Bhatiasevi said a group of experts, including mathematicians, epidemiologists and a virologists, were examining various measures countries could take to slow the spread of the disease.

Ms Bhatiasevi said school closures could be among the recommendations, but that it was up to each country to consider appropriate steps for their situations.

Experts predict that there will be a significant surge of new cases of swine flu in the northern hemisphere when the weather begins to cool in the autumn.

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